It is frequently necessary, particularly in temperate zones, to heat the water in a swimming pool. In days of cheap energy it was not economically impractical to provide a water heater that used cheap energy to draw water out of the pool, heat it, and re-introduce it into the pool. Such an arrangement is no longer economically practical and, in some jurisdictions, is even illegal.
Therefore recourse has been had to solar-powered water heaters usable with swimming pools. These arrangements can be divided into two main types, those that operate with the so-called greenhouse effect and those that operate without it.
The arrangements using the greenhouse effect normally circulate the water through one or more conduits provided in a closed chamber insulated from the surroundings and provided with a multilayer transparent window that allows short-wave radiation to enter the chamber and heat the water circulating through it. Such an arrangement can heat the water considerably, well above ambient, so that output temperature of 50.degree. C.-70.degree. C. are altogether possible.
These greenhouse-effect arrangements are, however, relatively expensive. The collectors are normally relatively large and complicated control arrangements and pumps must be provided. Their use, therefore, for heating swimming pools is relatively limited, as the first costs of the installation, even assuming that they can be operated for free, are very great. What is more they are relatively bulky and frequently constitute an eyesore at the swimming pool, which is obviously intended to be an agreeable recreational location.
The devices that operate without the greenhouse effect achieve a much lower output temperature, normally in the neighborhood of 20.degree. C.-30.degree. C. In these arrangements the fluid is passed through a conduit which is exposed directly to the rays of the sun, and which normally is not insulated at least on its upper side. For this reason the water cannot be heated much above ambient temperature, and in fact it is widely recognized that the lower the temperature differential the more efficient such a unit is. Thus it is advantageous to maximize the collector area and volume of flow through the collector so that the temperature differential between the feed end and return end of the collector is less than 4.degree. C.
To problem with such arrangements is, however, that the collectors once again are extremely bulky and unattractive. To effectively heat a swimming pool it is normally necessary to provide a collector surface area equal to at least half of the surface area of the pool whose water is being heated. The collectors are normally formed as panels having a length equal to approximately twice their width, so that it is necessary somewhere near the pool to erect an enormous battery of the panels to achieve the necessary heating effect. These panels have to, therefore, be connected together, normally in parallel. As a result considerable pains must be taken to ensure uniform flow through all of the collector panels of the battery. As with the greenhouse-effect heaters, it is therefore necessary to provide a large and unattractive heating installation as near the pool as possible, thereby once again making the pool surroundings relatively ugly and unsuitable for recreation purposes.
It has been suggested to provide a single extremely long tube that forms a spiral around the pool. Such an arrangement has, however, not been effective in practice as it is either necessary to provide a very large or long tube or a very large pump. Any of these solutions makes the device unworkable in other respects, so that this suggestion has never found a practical application.